A visitor of program elements, in the style of the visitor design
pattern. Classes implementing this interface are used to operate
on an element when the kind of element is unknown at compile time.
When a visitor is passed to an element's accept method, the visitXYZ method most applicable
to that element is invoked.

Classes implementing this interface may or may not throw a
NullPointerException if the additional parameter p
is null; see documentation of the implementing class for
details.

WARNING: It is possible that methods will be added to
this interface to accommodate new, currently unknown, language
structures added to future versions of the Java™ programming
language. Therefore, visitor classes directly implementing this
interface may be source incompatible with future versions of the
platform. To avoid this source incompatibility, visitor
implementations are encouraged to instead extend the appropriate
abstract visitor class that implements this interface. However, an
API should generally use this visitor interface as the type for
parameters, return type, etc. rather than one of the abstract
classes.

Note that methods to accommodate new language constructs could
be added in a source compatible way if they were added as
default methods. However, default methods are only
available on Java SE 8 and higher releases and the javax.lang.model.* packages bundled in Java SE 8 are required to
also be runnable on Java SE 7. Therefore, default methods
cannot be used when extending javax.lang.model.*
to cover Java SE 8 language features. However, default methods may
be used in subsequent revisions of the javax.lang.model.*
packages that are only required to run on Java SE 8 and higher
platform versions.